About one inch of snow (2.5cm) has fallen across many parts of central and southern England overnight, adding to the eight inches (20cm) that brought chaos to south-west England and south Wales on Tuesday evening.

In Devon, thousands of drivers were stuck in vehicles until the early hours and police escorted others to emergency rest centres while both Gatwick and Birmingham Airports have blocked runways, forcing delays and cancellations.

Motoring groups say the drastic rationing of grit will lead to accidents and long delays as the icy weather is expected to continue well into next week.

Pot holes have also appeared across the country as water enters cracks in the tarmac and then refreezes, causing the surface of the road to break up. Harrow Council, in north west London, estimated the repair bill at £2 million.

The Met Office said the overnight snow has spread north and eastwards from south-west England, covering an area bounded by Cheshire, Birmingham, London and west Kent.

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It said the snow would weaken to rain in most areas, but East Anglia could still see snow in the middle of the day.

January 13 is St Hilary's Day, which ancient folklore holds to be the coldest of the year, but daytime temperatures are forecast to edge up to 3C or 4C in most areas.

West Midlands Ambulance Service said Herefordshire and Worcestershire were worst hit by the fresh snow, while Warwickshire, Shropshire and the Black Country had also seen significant falls.

Herefordshire Council reported that the county had been hit by drifts of up to 6ft (1.83m) in some areas of high ground.

Lord Adonis, the Transport Secretary, announced fresh reductions after a meeting of the Government's Salt Cell, composed of departments and agencies with responsibility for gritting.

The emergency committee Cobra, which usually meets in times of national crisis, was also convened for the second time since the start of the big freeze.

Local authorities and the Highways Agency agreed to 25 per cent cuts on Friday, and the latest rationing will lead to an overall reductions in grit use of 40 to 50 per cent from normal levels.

To meet their targets councils will be forced to limit gritting to major A roads and key emergency and supply routes. They will also use less grit per square metre of tarmac.

Lord Adonis said that the rationing was necessary to ensure that major routes could be kept open for the length of the cold snap.

"If we did not act decisively reserves would run down fast, and essential roads would be at risk of closure if the freeze continues," he said.

Paul Watters, head of roads policy at the AA, said the rationing meant that all roads except motorways and trunk roads may now go unprotected.

"This is now a very serious situation, dire even. Put simply, it means that more roads will be dangerous, there will be more accidents, and drivers will have to be very patient with other road users."

The decision to stop gritting pavements has already led to a surge of accidents among pedestrians, with the London Ambulance Service reporting a doubling in "fall-related" calls over the past few days.

The death toll from the bad weather has now passed 30, with the latest casualties named as brothers Dinesh Dattani and Kishor Narandas who fell through a frozen lake in Leicestershire.

James Maw, 79, a pensioner from Sheffield, south Yorkshire, was also found dead at the bottom of an icy embankment after failing to return from a shopping trip.

The latest rationing was announced amid warnings that the gradual thaw is posing new problems for motorists in the form of potholes.

David Sparks of the Local Government Association (LGA) said that councils were working flat out to repair the "gaping sores in our road network" but needed the public to report any new holes.

Motoring groups have also warned that grit is being washed away from busy roads as ice thaws in daytime, forcing councils and transport agencies to start from scratch when freezing conditions return overnight.

All major roads were open yesterday apart from some routes over the Pennines. But several smaller roads remained closed due to difficult driving conditions, with Wales worst affected.

Rail and air passengers enjoyed less disruption to services thanks to the marginal improvement in conditions.

But the wintry weather continued to cause problems in the education system, with Ucas, the university admissions body, agreeing to postpone the deadline for applications after thousands of A Level students had their preparations for exams disrupted by the conditions.

Meanwhile, some householders have now gone five weeks without having their rubbish collected because their roads are impassable.

The Liberal Democrats also claimed that up to 1.7 million pensioners are not receiving cold weather payments of £25 a week to which they are entitled because they do not claim Pension Credit, which triggers the extra seasonal benefit.